The present invention generally relates to the operation of process control computers, and particularly relates to a method of resolving the selection of corresponding input signals received by each of a plurality of actively redundant process control computers.
In chemical manufacturing plants and other relatively large processing plants, a network of control computers and operator workstations may be needed to achieve automated control of an ongoing physical process in the plant. For example, the Jones et. al U.S. Pat. No. 4,663,704, issued on May 5, 1987, shows a distributed processing system for a plant in which a single data highway connects all the various input/output terminals, data acquisition stations, control devices, record keeping devices and so forth. Similarly, the Henzel U.S. Pat. No. 4,607,256, issued on Aug. 19, 1986, shows a plant management system which utilizes a plant control bus for the purpose of transmitting data to physical computer modules on the network.
In some of these process control computer networks, redundant process control computers are employed to enhance the reliability of the plant control and monitoring system. For example, the Fiebig et. al U.S. Pat. No. 5,008,805, issued on Apr. 16, 1991, shows a networked control system which includes a "hot standby" redundant processor that synchronously processes a control schedule table for comparison with control messages from a sender processor that are transmitted on the network. The redundant listener processor maintains a duplicate configuration in its memory ready to take over control of the system in the event of a failure of the sender processor. As another example, the McLaughlin et. al U.S. Pat. No. 4,958,270, issued on Sep. 18, 1990, shows a networked control system which employs a primary controller and a secondary controller. In order to maintain consistency between the primary data base and a secondary image of the data base, only predetermined areas changed are updated as a way of increasing the efficiency of the update function. Similarly, the Slater U.S. Pat. No. 4,872,106, issued on Oct. 3, 1989, shows a networked control system which employs a primary data processor and a back-up data processor. Normally, the back-up processor will be in a back-up mode of operation, and it will not operate to exercise control over the input/output devices or receive data concerning the states of the input/output devices. Accordingly, control over the input/output devices is exclusively carried out by the primary processor. However, the primary processor periodically transfers status data relating to its operation in the control of the input/output devices to the back-up data processor via a dual ported memory connected between the two processors.
In contrast with the above networked control systems, another control technique for redundant process control computers exists in which both of the process control computers operate on input data and issue control commands to the same output devices. This type of control technique may be referred to as active redundancy, because each of the redundant process control computers operate independently and concurrently on common input data. A discussion of this type of control technique may be found in the Glaser et. al U.S. Pat. No. 5,428,969, entitled "Process Control Interface System Having Triply Redundant Remote Field Units". This application is hereby incorporated by reference.
The use of active redundancy as a control technique presents a challenging problem where it is desired to arbitrate or resolve some or all of the input values for the process control computers, to the extent that differences in corresponding input values are found to exist. While each of the actively redundant process control computers will independently make process control decisions and generate their own output values, it may nevertheless be desirable to coordinate the input signals received by these actively redundant process control computers, so that the process control computers operate on the same input data. In this regard, it is possible for the actively redundant process control computers to receive corresponding or common input signals with different values.
These differences in input signal values may result from a variety of factors, such as a difference in the tolerance between the input circuits used to process an input signal from a single sensor. Additionally, a difference could arise from a transient condition associated with the transfer of the input signals across separate transmission paths. Furthermore, a fault in the reception, initial processing or transmission of an input signal could also cause a difference between corresponding input signals received at the actively redundant process control computers. It any event, it should be appreciated that it may be considered appropriate for each of the actively redundant process control computers to be aware of these situations, and that a collective response to these situation may well be inadequate.
Additionally, it should be appreciated that the coordination of input signals will raise complexities in both the ability of relatively independent process control computers to work together and the amount of processing time and effort required to enable this coordination to be realized.
Accordingly, it is a principal objective of the present invention to provide a method of individually and rapidly resolving the selection of corresponding input signals by each of a plurality of actively redundant process control computers.
It is another objective of the present invention to provide a method of input signal resolution which enables a front end determination to be made as to which input signal was selected by each of the actively redundant process control computers.
It is a further objective of the present invention to provide a method of input signal resolution for a plurality of actively redundant process control computers which includes a fault tolerant response to differences in magnitude between corresponding analog input signals.
It is an additional objective of the present invention to provide a method of input signal resolution for a plurality of actively redundant process control computers which has the capability of checking the validity of an exchanged set of input signals.
It is yet another objective of the present invention to provide a method of input signal resolution which enables input signals to be exchanged without interrupting the central processing units of the actively redundant process control computers.
It is also an objective of the present invention to provide a method of input signal resolution for a plurality of actively redundant process control computers which records a plurality of diagnostic signals during the arbitration process.